Characterization upon electrical hysteresis and thermal diffusion of TiAl3O x dielectric film
© Shi and Liu; licensee Springer. 2011
Received: 19 April 2011
Accepted: 19 October 2011
Published: 19 October 2011
In this paper, we have investigated the electrical properties of TiAl3O x film as electrical gate insulator deposited by pulsed laser deposition and presented a simple method to describe the thermal diffusion behaviors of metal atoms at TiAl3O x /Si interfacial region in detail. The TiAl3O x films show obvious electrical hysteresis by the capacitance-voltage measurements after post-annealing treatment. By virtue of the diffusion models composed of TiAl3O x film and silicon, the diffusion coefficient and the diffusion activation energy of the Ti and Al atoms are extracted. It is valuable to further investigate the pseudobinary oxide system in practice.
PACS: 77.55.-g; 81.15.Fg; 81.40.Gh.
High electrical permittivity (k) insulators are presently investigated as possible replacements of SiO2 or SiON film gate dielectric in order to hurdle the increased tunneling leakage current while the devices are further scaled down. Although a huge number of single-phase high-k materials have been investigated, there is no suitable material that could completely replace the traditional SiO2 as gate dielectrics [1–4]. It is exciting that the recent researches have been concentrated upon the pseudobinary oxides mostly. This section focuses on all these candidates in an attempt to combine and complement the desirable properties from different materials, and then overcome the deficiencies associated with the individual material. Furthermore, the criteria for possible replacement dielectrics require that those materials are chemically stable on silicon substrates at high temperature as well. In fact, most of these potential candidates might partially react with silicon substrates at the interface after thermal treatment, and thus cause an increase of leakage current and degrade devices performance with high-k gate dielectrics [5, 6]. As for a potential candidate, it is very important to pursue a small interfacial reaction for a perfect gate dielectrical candidate, except the preferable electrical properties.
In our earlier research, the electrical properties have shown that the TiO2-incorporated Al2O3 film may be a potential replacement as gate dielectrics because of its high dielectric constant, low leakage current, low charge density, etc. . Nevertheless, the interfacial reaction between the film and silicon substrate has been observed, despite of the improved device performance. The interfacial thickness, along with the leakage current, rises rapidly with the increase of the annealing temperature. This is very damaging for high-k gate material in electrical devices. In order to describe this behavior in detail, we have deposited the TiAl3O x films as much as micron of magnitude, and then, a longtime annealing treatment is adopted to drive all atoms in the film close to the balance state. Thus, the diffusion coupling model for the interfacial layer has been constructed, from which the atom diffusion properties may be extracted if the film is thick enough. In this paper, the TiAl3O x films have been deposited on silicon substrate successfully by pulsed laser deposition method, and characterized systematically by virtue of recording the electrical properties and the microstructure of the interfacial layer. With the diffusion coupling model of TiAl3O x film and silicon, the diffusion coefficient and the diffusion activation energy of the Ti and Al atoms are extracted. This is a part of effort to understand the TiAl3O x film in the future research.
Materials and methods
The pseudobinary TiAl3O x thin films were deposited by a pulsed laser deposition procedure with different thickness for different measure propose. Before the deposition, the substrates were ultrasonically cleaned by acetone and de-ionized water in turn, and then immersed in the diluted hydrofluoric acid solution to remove the native silicon dioxide, thus leaving a hydrogen-terminated silicon surface. The TiAl3O x films were grown on the treated silicon substrate at 673 K under a low oxygen partial pressure of 6.0 × 10-5 Pa. After the deposition process, some of the films were in situ annealed at 673 K for 20 min to reduce the defects in the films as possible. To characterize the electric properties of the TiAl3O x films with the capacitance-voltage (C-V) measurement, the corresponding films were deposited on the abovementioned available silicon substrates to form metal-oxide-semiconductor capacitor (MOS) structures with platinum electrode.
To further analyze the atom distribution at higher temperature, the films were post-annealed at 773, 873, 973, and 1,073 K for 6 h, respectively. High-resolution transmission electron microscopy (HRTEM, Tecnai G2F20 STEM, FEI Company, Hillsboro, OR, USA) with energy dispersive microanalysis system of X-ray (EDX) was used to investigate the interfacial structure of the cross-sectional samples and to determine the distribution of the fabricated diffusion samples in vertical section.
Results and discussion
Microstructural characteristics of the interface
Characterization on thermal diffusion
From the high-resolution transition electron microscopy image, there is an obvious interfacial layer at film/silicon interface after the course of atoms diffusion upon thermal treatment. Similarly, a satisfied interfacial layer could be obtained, in the condition of enough film thickness and post-annealing time. Therefore, the scanning points are chosen from the bulk of the film, until the silicon substrate. Thus, the stable concentration could be regarded as the diffusion source along the direction perpendicular to the substrate.
where D is the diffusion coefficient. Therefore, by the linear relationship between the concentration and the diffusion distance at a certain time, the behavior of the thin-film couples may be analyzed in detail.
In summary, with the C-V measurements, the electrical hysteresis of the TiAl3O x thin films has been revealed after high post-annealing temperature, which may be attributed to the oxide trapped charges density as much as 1012/cm2. The thermal diffusion behavior has occurred at the interfacial region after the thermal treatment, and therein, the interfacial layer thickness depends on the annealing temperature.
The thermal diffusion of metal atoms at TiAl3O x /Si interfacial region has been presented as well. The diffusion coefficient and the diffusion activation energy of the Ti and Al atoms derived from the films post-annealed at high temperature clearly disclose the diffusion behavior quantitatively.
This work was sponsored by the Fundamental Research Funds for the Central Universities (KA2340000008 and WK2340000014).
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